1. Field of the Invention
The invention relates to a process for the continuous preparation of 3-halopropylorganosilanes of the general structure (III): EQU R.sub.b H.sub.3-a-b X.sub.a SiCH.sub.2 CH.sub.2 CH.sub.2 Y (III),
where:
R is CH.sub.3, C.sub.2 H.sub.5, C.sub.3 H.sub.7, OCH.sub.3, OC.sub.2 H.sub.5 or OC.sub.3 H.sub.7, PA1 X is F, Cl, Br or I, PA1 Y is F, Cl, Br or I, and where PA1 a and b are each one of the numbers 0, 1, 2 or 3 and the sum a+b is 1, 2 or 3. PA1 R is CH.sub.3, C.sub.2 H.sub.5, C.sub.3 H.sub.7, OCH.sub.3, OC.sub.2 H.sub.5 or OC.sub.3 H.sub.7, PA1 X is F, Cl, Br or I, PA1 Y is F, Cl, Br or I, and where PA1 a and b are each one of the numbers 0, 1, 2 or 3 and the sum a+b is 1, 2 or 3, comprising reacting, as starting materials, an allyl halide with a silane carrying at least one H atom, PA1 wherein the starting materials are present in stoichiometric amounts or one of the starting materials is present in substoichiometric amounts, PA1 wherein the reaction carried out is a partial reaction of from 10% to 80%, on a molar basis, of the starting materials, based on either material, when both are present in stoichiometric amounts, or based on the substoichiometric material. Preferably, the partial reaction amounts to between 30% and 60%, based on either material, when both are present in stoichiometric amounts, or based on the substoichiometric components.
2. Description of the Background
3-Halopropylorganosilanes are important industrial intermediates in organosilane chemistry. The end products prepared from the 3-halopropylorganosilanes are employed as adhesion promoters in composite materials, e.g. in the paint and glass fiber industry, in founding and in the adhesives industry, further fields of application being elastomers and sealing compounds. Application examples are silanized glass fibers, particle reinforced plastics systems, silica-filled rubber articles, e.g. tires, the modification of hydroxyl-functionalized surfaces, substrate immobilization, silane polycondensation and constructional proofing compounds.
The preparation of 3-halopropylorganosilanes (III) is effected by preferably continuous catalyzed reaction of an allyl halide (II) with a silane (I) carrying at least one H atom and can be described by the following general reaction equation: ##STR1##
This hydrosilylation reaction is always accompanied by an unwelcome side reaction which can be described by a hydrogen-halogen exchange and which gives rise to the by-product propene (V): ##STR2##
The propene (V) formed in this reaction is able to react further with the organosilane (I) used to give the unwelcome by-product of a propylorganosilane (VI): ##STR3##
as a result of which the 3-halopropylorganosilane (III) yield of the preparation process is reduced in favor of the unwanted propylorganosilane (VI).
If X=Y=Cl, b=0 and a=3, this describes the preparation of 3-chloropropyltrichlorosilane (CPTCS) from trichlorosilane (TCS) and allyl chloride (AC) in analogy to the equations (1) to (3). This gives the reaction equation: ##STR4##
the side reaction equation ##STR5##
the secondary reaction ##STR6##
which produces the unwelcome propyltrichlorosilane (PTCS) from trichlorosilane and propene.
The reaction of allyl chloride with trichlorosilane is performed in the liquid phase, usually either on a heterogeneous catalyst or in the presence of a homogeneous catalyst.
DE 41 19 994 A1 discloses a process for preparing 3-chloropropylsilanes, in which the formation of the unwelcome by-product propylsilane is reduced, involving complex process management, by means of a preferably at least three-fold molar excess of allyl chloride. In the process, the reaction mixture containing at least allyl chloride is evaporated, condensed and then together with optionally added hydrosilane, passed over a catalyst, a stoichiometric excess of allyl chloride being maintained in the feed to the catalyst.
DE 34 04 703 A1 discloses a process for the preparation of 3-chloropropyltrichlorosilane (CPTCS) with improved yield. This involves carrying out the reaction of allyl chloride and trichlorosilane with special highly selective Pt catalysts in accordance with DE 34 04 702 A1 with simultaneous separation of propene, resulting in improved selectivity for CPTCS. The improved selectivity is essentially achieved by the use of the very expensive, special hydrosilylation catalysts according to DE 34 04 702 A1. While conventional processes require a high excess of trichlorosilane to achieve complete reaction of allyl chloride, DE 34 04 703 A1 achieves complete reaction of allyl chloride even with a small excess of trichlorosilane.
However, said excess trichlorosilane promotes, with respect to propene still present, the secondary reaction to the unwelcome by-product propyltrichlorosilane, which is also true, in general terms, for the preparation of 3-halopropylorganosilane with the unwelcome by-product propylorganosilane (VI).